1. Technical Field
The invention relates to blends of epoxy resins and copolymers of styrene and maleic anhydride. In one of its aspects, the invention relates to blends of epoxy resins that can serve as polymer matrices in composite materials and laminates such as multi-layer printed circuit boards and communications antennae.
2. Discussion
Multilayer printed circuits find use in a variety of electronic applications. For example, they furnish structure for mounting semiconductor components, and provide the primary interconnection between discrete components in most computing systems such as desktop and laptop computers. These circuits continue to evolve in complexity and density as semiconductors and systems become more complex. More advanced multilayer printed circuits are referred to as Multichip Modules, High Density Interconnects, Micro Via Multilayers, and PCMCIAs.
Copper clad laminates provide the electrical insulation and the physical structure for each of the individual circuit layers of a multilayer printed circuit. In some cases, the copper clad laminates themselves function as application components, such as passive capacitors, as in ZYCON BURIED CAPACITANCE(trademark) technology, for example. Conventional copper clad laminates are comprised of three main componentsxe2x80x94copper foil, thermosetting resin, and woven or non-woven fabric reinforcement. Generally, the copper foil is about 35 microns thick and is made by an electrodeposition process. Often, one or both sides of the copper foil are treated to improve adhesion to the fabric reinforced thermosetting resin.
A conventional copper clad laminate is typically manufactured using a three-step process. In a first step, a woven fabric is coated or impregnated with a thermosetting resin, forming what is known as a prepreg. To improve handling and bonding, the prepreg is usually partially curedxe2x80x94or B-staged. In a second step, the prepreg is interposed between two copper foils, and in a third and final step, the prepreg and the copper foils are bonded together with heat and pressure to form the copper clad laminate.
Typically, epoxy resins are employed as the thermosetting resin in copper clad laminates. Epoxy resins offer many advantages over competing systems, including low cost, relatively high glass transition temperature, Tg, and well understood cure chemistry. However, in certain high frequency applications (greater than one MHz, for instance), the use of epoxy resins leads to unacceptable signal loss because of their relatively high dielectric constant, Dk, and dissipation factor, Df. Furthermore, many epoxy resin systemsxe2x80x94notably those cured with dicyandiamidexe2x80x94exhibit moisture and heat sensitivity. Such epoxy resin systems often cannot withstand the higher temperatures and longer residence times associated with lead-free soldering processes.
The invention provides a resin blend that possesses many of the advantages of epoxy resins, but also exhibits lower dielectric constant and dissipation factor, and improved moisture and thermal resistance. The resin blend includes a copolymer of styrene and maleic anhydride (SMA), an epoxy resin, and a multifunctional amine cross-linking agent. The cross-linking agent generally contains at least two primary amino groups to promote formation of imide functionalities upon reaction with the anhydride moieties of the SMA copolymer. Generally, the epoxy resin can be a brominated or a phosphonated resin. In a preferred embodiment, the epoxy resin is a brominated resin, the SMA resin has a 4 to 1 weight ratio of styrene to maleic anhydride, and the cross-linking agent is 2,4-diamino-6-phenyl-s-triazine. The resin blend can be used as a polymer matrix in composite materials, as an impregnating resin in prepreg and as an impregnating resin in metal clad laminates. Compared to conventional SMA copolymer/epoxy resin blends, the resin blend according to the invention exhibits higher thermal and moisture resistance and lower dielectric constant and dissipation factor, making it especially useful in lead-free soldering and high-speed, low-loss printed wire board applications.
The invention relates to blends of epoxy resins and copolymers of styrene and maleic anhydride (SMA) that are cross-linked with multifunctional amine compounds. The blends can serve as polymer matrices in composite materials and laminates. Compared to conventional SMA copolymer/epoxy resin blends, the disclosed resin system exhibits higher thermal and moisture resistance and lower dielectric constant and dissipation factor, making it especially useful as an impregnating resin in high-speed, low-loss printed circuit boards. In addition, the disclosed resin system should be more robust than conventional epoxy resins in lead-free soldering processes in the manufacture of printed circuit and wire boards, which require high temperatures and long residence times (see Table 2 for Solder Float @ 300xc2x0 C. and T300 tests).
The resin blend generally comprises about 10 to 90 wt % SMA copolymer, about 90 to 10 wt % epoxy resin, and about 1 to 10 wt % multifunctional amine cross-linking agent. When cured, the SMA copolymer produces a rigid, low dielectric material with superior thermal stability and moisture resistance. SMA copolymer is commercially available in a broad range of molecular weights and monomer weight ratios. Typically, the molecular weight of the SMA copolymer can range from about 1400 to about 10000 (weight average molecular weight), and the weight ratio of styrene monomer to maleic anhydride can range from about 1:1 to about 8:1.
The epoxy resin component of the blend is included, in part, to reduce brittleness of the cured resin system. Epoxy resins are characterized by the presence of a three-member ring known as an epoxy, epoxide, oxirane, or ethyoxyline group. The disclosed resin blend can employ any number of epoxy resins used in the manufacture of composite materials and laminates. Examples of useful epoxy resins include diglycidyl ethers of bisphenol A, bisphenol F, aliphatic glycols, novolacs or other polyols; multifunctional cresol-novolac resins; epoxy phenol novolac resins; polynuclear phenol-glycidyl ether-derived resins; cycloaliphatic epoxy resins; and aromatic and heterocyclic glycidyl amine resins. The latter resins include tetraglycidylmethylenedianiline-derived resins, triglycidyl p-aminophenol-derived resins, and triazine-based resins such as triglycidyl isocyanurate. In printed circuit board applications, one or more of the epoxy resins are typically brominated and/or phosphonated.
An important feature of the curing system is the formation of thermally stable imide groups. Hence, the multifunctional amine cross-linking agent or agents ordinarily include at least two primary amino groups, which promotes the formation of substituted-succinimide functional groups upon reaction of the cross-linking agent with anhydride moieties of the SMA copolymer. Thus, the cross-linking agent can be generally represented by the formula:
R1(NH2)n. xe2x80x83xe2x80x83I 
In formula I, n is an integer greater than or equal to two and R1 is aliphatic, alicyclic, aromatic, and arylalkyl, and can contain heteroatoms such as oxygen and nitrogen.
Because of their thermal stability, triazine-centered multifunctional amines are especially useful cross-linking agents and can be represented by the formula: 
In formula II, R2 is aliphatic, alicyclic, aromatic, arylalkyl, or hydrogen, and can contain heteroatoms such as oxygen and nitrogen. Particularly useful examples of cross-linking agents represented by formula II include benzoguanamine (2,4 diamino-6-phenyl-s-triazine); alkyl guanamines such as acetoguanamine; imidizole-substituted guanamines; and melamine (2,4,6 triamino-s-triazine). Other useful multifunctional amines include derivatives of dianiline having the formula: 
where, R3 is aliphatic, alicyclic, aromatic, arylalkyl, or hydrogen, and can contain heteroatoms such as oxygen and nitrogen. Examples of cross-linking agents
represented by formula III include alkylene dianilines such as methylene dianiline, and phenylene ether dianilines.
During cure, the amino groups will react with the anhydride moieties of the SMA copolymer as well as the oxirane groups of the epoxy resin. In addition, the anhydride moieties of the SMA copolymer, as well as residual hydroxyl groups of the epoxy resin (and phenolic hydroxyl groups of any novolac resin present in the blend), will react with the oxirane groups forming a complex polymer network. However, the reaction between the amino groups and the anhydride groups should predominate, ensuring formation of imide groups in the polymer network. To achieve good prepreg and varnish shelf life and to maintain good mechanical and hydrophobic properties, the ratio of molar equivalents of anhydride to amino groups should range from about one to about five, depending on the amount of epoxy (molar equivalents) in the resin blend. Excess anhydride reacts with the oxirane groups of the epoxy resin to provide good laminate properties.
In manufacturing composite materials and laminates, the uncured resin system typically includes diluents or solvents to reduce the viscosity of the resin blend. One may employ any number of solvents generally used with epoxy resin systems, as long the solvents are compatible with the SMA copolymer and the multifunctional amine cross-linking agent. An especially useful solvent system includes xcex3-butyrolactone (GBL), propylene glycol-methoxy ether acetate (PMA), and methyl ethyl ketone (MEK) combined in about a 9:5:1 weight ratio.
In addition to diluents that aid processing, laminates such as printed circuit boards typically include reinforcement fillers. Thus, the disclosed SMA copolymer and epoxy resin blend will generally include solid fillers. Useful fillers include talc, mica, clay, silica, TiO2, BaTiO3, boron nitride, glass fiber, aramid fiber, polyphenylene oxide (PPO), and thermoplastics, among others.